This invention relates to a capacitor element, and more particularly to a capacitor having parallel plates with a side length of more than about one fourth of a wavelength of a signal used in a microwave integrated circuit.
In conventional microwave circuits, distributed element circuits or lumped element circuits may be used. In a distributed element circuit, parameters such as resistance, inductance and capacitance are distributed as in a transmission line, where the capacitance exists between the conductors, and the resistance and inductance are due to the conductors themselves. In contrast, in a lumped element circuit, these parameters are lumped as used in a gate or source circuit of a GaAs FET, in which the capacitance exists between a conductor and the ground, and these parameters can be treated for circuit analysis as a single inductance, capacitance, resistance, etc., when the frequency is sufficientely low. However, when the frequency used in a lumped element circuit is relatively high, especially when the size of the electrode of the capacitor is greater than or equal to about one fourth of the wavelength, the effect of size on reactance must be considered. That is, the reactance of the capacitor changes from capacitive reactance to inductive reactance. Therefore, the lumped element circuit, when used at such a high frequency, behaves as a distributed element circuit, and, accordingly, the designed values of the parameters become different from the actual values, as will be explained hereinafter in detail.